Apparatus for drying of granulated polyamides



1963 w. ZEHNDER 3,112,188

APPARATUS FOR DRYING 0F GRANULATED POLYAMIDES Filed Dec. 14, 1959 INVENTOR.

WALTER Z EH/VDER BY & 42%);

A TTORN Y5 ilnited States Fatent 3,ll2,l%8 Patented Nov. 26, 1963 has 3,112,188 APPARATUS FOR DRYING F GRANULATEB POLYAMEBES Walter Zehntier, Ems, Switzerland, assignor to invents! AG. fuer Fcrschung und Patentverwertung, Zurich, witzerland Filed Dec. 14, 1959, Ser. No. 859,280 Claims priority, application Switzerland Dec. 20, 1%8 2 Claims. (Cl. 34-77) The invention relates to a novel process of drying granulated polyamides, more particularly to a continuous process which requires no vacuum, and to a device to carry out that process.

The changeover to continuous operation using the drying installations for polyarnide granulate which heretofore have customarily been employed in the art oilers considerable diflicu lties. Up to now, only vacuum driers have been proposed land to date there are no reliable and absolutely vacuum-tight feed and discharge means available which prevent the penetration of air which has a harmful effect in very small quantities under certain conditions.

It has now been found that it is possible to dry polyamide gnanulate continuously in an economically advantageous manner by causing the wet granule to slide, at temperatures below its melting point, through a drying tower which is broader at least in its upper part than in its lower part, while drying gas inert to polyamide is introduced from below at the drying temperature.

The preferred inert drying gas is pure nitrogen, which acts simultaneously as a protective gas, as conveying or entnaining medium for the water to be removed and as heat-transmitting medium. The heat transmission to the chips is considerably better, and much shorter drying times are required than in the case of drying in vacuo. In principle, the process is a continuous counter-current drying process, the chips being introduced at the top of the drying tower and leaving it at the bottom, while dry inert gas, heated to the drying temperature, is blown in at the bottom and emerges at the top charged with moisture. The wet chips are dried in at least :two stages. The bulk of the Water is removed in the preliminary drying stage and, consequently, the main proportion of the quantity of heat required also must be introduced here, i.e., a large quantity of inert gas is necessary in this stage. To prevent excessive gas velocity and pressure drop of the inert gas, the drying tower has small height with a relatively large diameter in this stage. As a uniform flow of the chips does not matter in stage, the adhering water and the major portion of the water contained in the chips can be removed comparatively easily, any irregularities being adjusted in the last stage. 3

In principle, the drying can be carried out at any temperature below the melting or softening point, but the possibility of after-condensation at excessively high tempenatu-res, i.e., at temperatures above 200 C., should be taken into account.

Generally, it sufiices to dry in two stages, i.e., the first stage is followed immediately by the final stage, but, under certain conditions, it may be advantageous to interpose one or more intermediate stages between the first and the last stage in such manner that the height or depth of the layer of polyamide chips increases from stage to stage and its diameter decreases. In general, nitrogen is used as drying gas, but in prmciple any gas which is inert to- Wards the poly amide under the conditions of the process may be employed.

Contrary to the views heretofore held, it has been found that polyamides can also be dried with air if the drying by the process according to the invention is carried out at temperatures at which yellowing does not yet take place, generally below C. Unless extreme drying (water content 0.l%) is to be accomplished, no substantial yellowing of the chips occurs.

The drying gas may, of course, be circulated. To obviate excessive accumulation of steam in the gas stream, part of the stream is separated directly after leaving the tower, so that the bulk of the water can be eliminated and separated. The gas then is i e-heated to drying temperature and intnoduced into the last stage. In this stage, the final drying is carried out with the gas freed from water. The gas then flows upward through the column and enters the first stage.

The entire plant is advantageously operated at a slight excess pressure so that no atmospheric air can enter. A small quantity of fresh gas is blown through the plant for this purpose. A corresponding quantity of gas issues through the chipcharging tube and also prevents the penetration and entrainment of atmospheric air at this point. The chips do not reach the temperature required for driving out the last traces of water until the end of the drying process and are kept at that temperature for only a short time, whereby thorough drying under the mildest conditions is facilitated.

The invention will now be more fully described with reference to the accompanying drawing which is a flowsheet of the operation. The description then will be augmented by a number of examples. However, it should be understood that both drawing and examples are given merely by way of illustration, not of limitation, and that numerous changes may be made in the details without depanting from the spirit and the scope of the invention as hereinafter claimed.

Referring now to the drawing, the chips pass through tube 1 into the plant, travel through stage I and immediately thereafter through stage II and through succeeding stage III and are discharged again at 2. The drying gas is circulated by means of fan 3. Its main quantity flows through pipes 4 and 5' to the gas heater 6 and is fed from there through pipe 7 and the annular passage 8 to the first stage of the dryer. After flowing through that stage, the gas travels back to fan 3 by way of pipe 9. A partial flow passes through pipes 4 and Ill into the heat exchanger 11 and thence into cooler 12. Here, the major portion of the water is extracted and flows, concurrently with the gas, through pipe 13 into separator 14, is separated from the gas and drawn elf through pipe 15. The gas flows through pipe 16 to the other end of the heatexohange surface of the exchanger 11 and thence through pipe 17, heater 18, pipe 19 and the annular passage 20 into the bottom of the second stage of the drying column, and through pipe 19a into the bottom of the third stage, flows through these stages and then through the first stage and b ack to fan 3 by way of pipe 9. A small stream of fresh gas is fed into the installation through pipe 21 and a corresponding quantity of circuit gas leaves the installation through pipe 1.

As shown in the drawing, section II is narrower and longer than section I. Section III is narrower and and longer than section II, the actual length not being shown in the drawing.

Example 1 Poly-e-caprolactam chips having a grain size of 2.5 mm. are dried in a plant :as described above. The first stage has a layer depth of 600 with a diameter of 209 mm. and the second stage has a diameter of 150 mm. and a length \of 1,800 mm. 2 kg./ h. of dry chips are discharged at the bottom through the discharge means, which corresponds toga dwelling time of approximately 16 hours.

The quantity of nitrogen supplied to the first stage is 40 m. /h. at C. and that supplied to the second 3 stage is 8 m. /h. at 110 C. The gas supplied to the second stage is cooled to 20 C. and the extracted water is separated. The quantity of fresh nitrogen is 0.7 111 /11. to chips dried in this manner have a water content of 0.11%.

Example 2 Poly-E-caprolactarn chips are dried in the same plant as in Example 1. The quantity of chips discharged is 1.4 kg/h. The quantity of gas supplied to the first stage is 30 mi /h. nitrogen at 125 C. and that supplied to the second stage 10 m. /h. at 130 C. This latter quantity of gas had previously been cooled to 5 C. and the extracted Water separated. The quantity of fresh nitrogen is 0.7 m. /h. The chips have a final water content of 0.04%.

Example 3 Poly-e-caprolactam chips are dried in the same plant as in Example 1. The quantity of chips discharged is 1 kg./h. The quantity of air supplied to the first stage is 25 m. /h. at 70 C. and the quantity of air supplied to the second stage is mfi/h. :at 82 C. Then entire quantity of air is sucked in from the atmosphere through a filter and is blown cit again into the atmosphere at the top of the drying plant. The air intended for the second stage is compressed to 10 atmospheres excess pressure and cooled to C., the water is separated and the air is again expanded. The water content of the chips is 0.12%.

When caprolactam chips thus dried were injectionmolded, flawless pieces were obtained indicating that the chips had not been impaired by the drying process but had in fact been improved.

I claim as my invention:

1. A device for the continuous drying of granulated polyamides, which comprises, in combination, a drying tower subdivided into three vertical compartments, each lower compartment having lesser diameter and greater height than the preceding one; charging means for said polyamide on top of said tower; discharge means for said polyarnides on the bottom of said tower; gas inlet means near the bottom of each compartment of said tower; gas outlet means near the top of said tower; and cooling and circulating means for said gas, connecting said gas inlet means near the bottom of the lower two compartments with said gas outlet means near the top of said tower.

2. A device for the continuous drying of granulated polyamides, comprising, in combination, a drying tower subdivided into three vertical compartments, each lower compartment having lesser diameter and greater height than the preceding one; a charge opening for said polyamides at the top of said tower; a discharge outlet for said polyamides on the bottom of said tower; gas inlet lines near the bottom of each compartment of said tower; at gas outlet line near the top of said tower; one gas heater disposed within the gas inlet line for the uppermost compartment, and one gas heater within the line serving the lower compartments; a heat exchanger and a cooler connected to each other and interposed between the gas inlet lines and the gas outlet line; said cooler serving to remove water, entrapped during drying, from the gas from said outlet line; means for disposal of said water; and a fan interposed between said gas outlet and gas inlet line facilitating circulation of the gas through inlet lines into said compartments, through said outlet line and through said cooler and heat exchanger, back into said compartments by way of said inlet lines.

References Cited in the file of this patent UNITED STATES PATENTS 1,277,895 Foster Sept. 3, 1913 1,554,854 Hubmann Sept. 22, 1925 2,077,346 Voskamp Apr. 13, 1937 2,316,195 Troxler Apr. 13, 1943 2,475,984 Owen July 12, 1949 2,560,356 Liedholm July 10, 1951 2,577,915 Piller ct al Dec. 1], 1951 2,716,289 Lauck Aug. 30, 1955 FOREIGN PATENTS 568,856 Italy Nov. 9, 1957 

1. A DEVICE FOR THE CONTINUOUS DRYING OF GRANULATED POLYAMIDES, WHICH COMPRISES, IN COMBINATION, A DRYING TOWER SUBDIVIDED INTO THREE VERTICAL COMPARTMENTS, EACH LOWER COMPARTMENT HAVING LESSER DIAMETER AND GREATER HEIGHT THAN THE PRECEDING ONE; CHARGING MEANS FOR SAID POLYAMIDE ON TOP OF SAID TOWER; DISCHARGE MEANS FOR SAID POLYAMIDES ON THE BOTTOM OF SAID TOWER; GAS INLET MEANS NEAR THE BOTTOM OF EACH COMPARTMENT OF SAID TOWER; GAS OUTLET MEANS NEAR THE TOP OF SAID TOWER; AND COOLING AND CIRCULATING MEANS FOR SAID GAS, CONNECTING SAID GAS INLET MEANS NEAR THE BOTTOM OF THE LOWER TWO COMPARTMENTS WITH SAID GAS OUTLET MEANS NEAR THE TOP OF SAID TOWER. 